Characterization of Soluble Factors of Immunosuppressor T Cells. Antisera are being produced against the I-J region of the major histocompatibility complex (MHC) of each of the two congeneic mouse strains, B10A(3R) and B10A(5R), to establish if suppressor T-cell factor(s) detected in our tumor systems are by-products of the I-J region of the MHC. Immunological Mechanisms in Tumor-Bearing Hosts. Recent experiments in various tumor systems (guinea pig sarcoma, mouse myelomas and mouse sarcoma) revealed that suppressor and killer cells co-exist in tumor-bearing hosts. It is proposed to investigate the interaction of these lymphoid cell populations with the tumor and with one another in normal and in X-irradiated and bone-marrow reconstituted recipients. Attention will be also paid to the possible effect of humoral factors such as soluble antigens, antibodies and immune complexes present in tumor-bearing hosts. Immunological Studies on Gastrointestinal Tumors. Tumor antibodies have been detected in patients with gastrointestinal carcinomas by a solid phase radioimmunoassay developed in this laboratory. Approximately 40% of the patients had antibodies to crossreactive antigen(s) which is not identical with CEA or blood group substances. It is proposed to investigate the class of antibodies involved and to establish if there is any relationship between antibody production and survival of the patients. Moreover, circulating antigens will be determined in the patients' plasma with the solid phase method developed in this laboratory. The Use of Antibodies as Carriers of Cytotoxic Drugs to Tumor Foci. Conjugates of daunomycin with anti-guinea pig fibrin antibodies have been shown to be capable of destroying the guinea pig MC-D sarcoma in vivo. The cellular mechanism underlying the observed tumor destruction will be studied by adoptive transfer techniques using X-irradiated recipients. In addition, improved methods for the coupling of cytotoxic drugs with antibodies and for the purification of tumor-specific antibodies will be explored.